Analogue to digital converters are used in many applications where it is desirable to convert an analogue value (such as, for example, an analogue voltage) to a digital value. The digital value may then, for example, be processed using digital processing.
Many analogue to digital converters (ADCs) use a digital to analogue converter (DAC). A digital value stored within the ADC is converted to an analogue voltage using the DAC. The resulting analogue voltage is then compared with the analogue input voltage using an analogue comparator. The output of the comparator, which indicates whether the analogue output of the DAC is higher or lower than the analogue input voltage, is then used in an analogue to digital conversion to modify the digital value, until the output of the DAC is substantially equal to the input voltage. At this point, the digital value represents the analogue input voltage, and is provided as the output from the ADC.
The digital value may be modified using a method of successive approximation. In an example of this method, the bits of the digital value are set to 0, except for the most significant bit (MSB) which is set to 1. If the output of the comparator indicates that the output of the DAC is too high, and thus the digital value is too high, then this bit is reset to 0, otherwise it remains at 1. The process is then repeated for the next most significant bit, and so on, until all the bits of the digital value have been considered. The digital value then substantially represents the analogue input voltage. This approach has the advantage that a single conversion requires the same time and/or number of clock cycles, regardless of the magnitude of the input voltage.
A number of different DAC architectures can be used within an ADC. A string DAC, for example, has a string of (2n−1) resistors connected in series between a power supply voltage and ground, where n is the number of DAC bits. A number of switches are employed to connect the output of the DAC to a point along the resistor string, depending on the value of the digital input to the DAC. The resistor string acts as a potential divider, and the output, therefore, has an appropriate voltage level that represents the digital input to the DAC. A string DAC has good accuracy, as it is possible to produce a number of highly matched resistors on an integrated circuit, and can operate at high speeds. However, the large number of resistors in a string DAC may introduce high levels of noise into the output of the DAC. The number of resistors in a string DAC can be reduced by forming a segmented DAC by cascading two or more string DACs that have fewer resistors in each respective resistor string, at the possible expense of speed and/or accuracy.
It is an aspect of embodiments of the invention to at least mitigate one or more of the problems of the prior art.